Helical antennas have widely been used for mobile and satellite radio applications since the 1950s [see Kraus reference listed below]. Compared to monopole antennas, helical antennas are preferred for their high gain and wideband impedance characteristics despite their compact form. In addition, helical antennas offer wideband circularly polarized (CP) radiation patterns and simple periodic structures. Helical antennas have different modes of operation. The helix is operating in the axial mode when the circumference in free space wavelength of the helix is about one wavelength. The principal lobe of the radiation pattern of an axial-mode helical antenna is extended along its axis [see Kraus reference listed below].
Several variations of the axial-mode helical antennas exist in the literature that focus on optimizing the length, pitch angle or radius of the helical antenna for a certain application. In one example [see Killen reference listed below], the pitch angle of an axial-mode helical antenna is varied in a non-linear manner from a relatively small angle at the feed to a large angle at the distal end of the antenna, to optimally match the phase velocity of the EM wave travelling through the antenna to that of the free space, and to provide multiple peak gains. In another example [see Chen reference listed below], exponential pitch spacing is recommended to increase the CP bandwidth of the antenna. In another example, a spring tunable helical whip antenna is built in [see Wilson reference listed below] for mounting in the frame of a vehicle. In addition, a Tri-band helical antenna to cover the EGSM/GPS/PCS bands is designed in another example [see Zhang reference listed below] that includes a dual-pitch axial-mode helical antenna. A variety of increasing cone, decreasing cone, and envelope helices are also introduced in further examples [see Kraus reference listed below]. However, little attention has been paid to dynamic optimization of the helix antenna parameters to match real-time application requirements.
On the other hand, emerging wireless communication devices call for antennas that can dynamically adjust one or multiple antenna characteristics such as the far-field radiation pattern, centre frequency or directivity, to new operating conditions. For example, such reconfigurable antennas can dynamically change their radiation pattern in order to improve the transmitted power efficiency and therefore conserve the battery of a hand-held device or dynamically steer nulls in the radiation pattern to mitigate unwanted interference and increase the signal-to-noise ratio (SNR) of a noisy link.
The adjustment of antenna characteristics can be realized through electrical, mechanical or other means. Solid-state switches such as PIN diodes [see Roscoe reference listed below] and RF-MEMS switches [see Kiriazi reference listed below] are among the most common methods used [see Bernhard book reference listed below]. However, these methods typically suffer from disadvantages such as non-linearity and low isolation and therefore may be undesirably limited in their potential throughput. In addition, only certain discrete changes can be attained using these methods.
Mechanical approaches to reconfigure the antennas are in general slow but may deliver the most dramatic antenna parameter changes [see Bernhard book reference listed below]. In addition, since the changes by mechanical approaches are applied to the physical antenna structure, reconfigurability schemes may be attained that may not be possible by other methods.